1. Field of the Invention
The present invention relates to optical switches and, in particular, to Mach-Zehnder fiber switches.
2. Description of the Related Art
Optical switches are well known in the art, and are anticipated to have extensive applications in fiber optic communication and sensor systems. A particularly preferred kind of optical switch is an all-optical switch, wherein the actual switching of light from one output port to another output port is accomplished without mechanical or electrical switching, but rather by the input of an optical pump signal from a switching light source.
In the basic configuration of a Mach-Zehnder interferometer switch, an optical signal is provided as an input signal to the switch. The input signal is divided at an optical coupler and is caused to propagate in substantially equal portions in two arms of an interferometer. The two portions are recombined at a second coupler, and the two output ports of the second coupler are the two output ports of the switch. If the light portions recombining at the second coupler are in phase, the two portions constructively interfere at one output port of the second coupler and are output at that output port. If, on the other hand, the two light portions are not in phase, and in particular, if the two light portions incur a .pi. differential phase shift, the two light portions combine constructively at the other output port of the second coupler and are output at that output port. By controlling the differential phase difference between the two light portions, the optical signal can be selectively switched to be output at either of the two output ports.
The control of the differential phase shift is accomplished by inputting an optical pump signal into the interferometer switch. The optical pump signal has a different wavelength than the optical signal, and, by using a wavelength dependent multiplexing coupler, the pump light is caused to propagate in only one of the two arms between the two couplers. The pump light changes the propagation characteristics of the arm in which the pump light is propagating and causes the signal light portion propagating in that arm to incur a phase shift different from the phase shift of the signal light portion propagating in the other arm. By selecting the intensity of the pump light, the phase shift can be set to be equal to .pi. and the output light is switched between the two ports, as discussed above. The pump light can be selectively turned on or turned off to cause the signal light to be switched or not to be switched. More details regarding the operation of the pump signal in causing the switching will be discussed below.
A number of problems have been encountered in known interferometric switches. One problem has been to be able to construct the interferometer such that the two signals have a stable phase difference of 0 or .pi. in the absence of the pump signal. Another problem is that the refractive indices of the two fiber arms vary with temperature, and, if the temperature variations of the two arms are not equal or if the optical paths of the two arms are not equal, the temperature variations cause variations in the differential phase shift. Furthermore, the temperature variations induced by the pump can be different for the two arms because the pump signal propagating in one arm heats up that arm, while the other arm is affected less or not at all by the heat from the pump signal.
Certain temperature-stable Mach-Zehnder interferometers have been developed; however, such interferometers have not heretofore been suitable for use as an all-optical fiber switch using the intrinsic Kerr effect of the fiber. This is because the small size of these temperature-stable Mach-Zehnder interferometers allows for only a very short fiber length between the two fused couplers. Thus, in order to induce the required .pi. differential phase shift over a very short fiber arm length using the Kerr effect, a very high pump power is required, which will induce catastrophic breakdown of the optical fiber. Thus, there continues to be a need for a temperature-stable Mach-Zehnder interferometer switch with a strong nonlinearity.